1. Field of the Invention
The invention generally relates to electrochemical cells for use in powering implantable devices and in particular to an improved current collector material for use within electrochemical cells having solid cathodes employing polycarbon monofluoride as an active cathode material.
2. Description of the Related Art
A wide range of electronic devices are designed for surgical implantation into humans or animals. One common example is the cardiac pacemaker. Other examples of implantable devices include devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissues.
Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, memory devices, or other large scale integration (LSI) devices. Often, the devices are designed for transmitting signals to remote sensing devices. With the increase in the sophistication of implantable devices and in particular with the need to reliably transmit signals to sensors external to the body, the need for improved power cells for powering the implantable devices has increased greatly. There are, of course, limitations on the design and configuration of power cells for use in implantable devices, especially with regard to the size and shape thereof. Moreover, the power cells for the implantable devices must be highly reliable and be capable of providing an adequate amount of current and voltage for an extended period of time.
One type of power supply for use in an implantable device is an electrochemical cell. Examples include cells employing lithium as an anode material and polycarbon monofluoride as an active cathode material. Typically, within such cells, a metal foil anode coated with lithium is provided in combination with polycarbon monofluoride formed on a current collector. A polymeric separator is positioned between the anode and the cathode thereby forming an electrode structure. The electrode structure is mounted within a cell housing which is flooded with a liquid non-aqueous electrolyte. Appropriate electrical contacts are provided to the anode and cathode.
Within electrochemical cells employing polycarbon monofluoride the choice of current collector material is particularly important. The current collector material must have electrical characteristics sufficient to achieve adequate current collection. The current collector material must also have certain mechanical characteristics to allow for proper formation of the electrochemical cell. For example, within button cells, such as those commonly employed within watches, the base portion of the button cell is typically employed as a current collector. Accordingly, the material chosen for the current collector must be sufficiently sturdy to resist bending or deformation which could damage the cell. Moreover, button and cylindrical cells include a crimped gasket around the perimeter of the cell. Hence, the current collector material must be sufficiently malleable to allow for formation of the gasket seal while also being sufficiently rigid to prevent subsequent damage to the seal which could result in either a loss of internal electrochemical cell materials or a short circuit between the collector base of the cell and the anode of the cell. The current collector material most commonly employed in button and cylindrical cells is "446-stainless steel", also known under the trade name Shomac.
Conventional stainless steel is not employed as a current collector in electrochemical cells having polycarbon monofluoride. Conventional stainless steel includes a considerable percentage of nickel. The polycarbon monofluoride reacts with the nickel causing an erosion of the stainless steel and a loss of active cathode material. 446-stainless steel, however, does not include any nickel but instead includes chromium and molybdenum and hence resists erosion. 446-stainless steel can be quite expensive and the need to employ 446-stainless steel within electrochemical cells therefore increases the overall cost of the cells. Accordingly, it would be desirable to provide an improved polycarbon monofluoride electrochemical cell which can employ conventional stainless steel rather than 446-stainless steel.
Another current collector material commonly employed is titanium. Titanium has good electrical characteristics for collecting current. However, titanium can be fairly expensive, thereby increasing the overall cost of the electrochemical cell. Moreover, titanium is not adequately annealable. Hence, for certain electrochemical cell configurations, titanium cannot easily be employed.
Accordingly, it would be desirable to provide an improved current collector material for use in electrochemical cells employing polycarbon monofluoride as an active cathode material. It is also a particular object of the invention to provide improved current collector materials for use within electrochemical cells of the type employed within implantable medical devices.